This invention relates to a method and apparatus for controlling an X-ray generator to obtain a relatively constant output window brightness from a directly viewed X-ray image intensifier tube.
When an X-ray image intensifier tube is used in a fluoroscopic mode, the output screen is sometimes directly viewed and is therefore exposed to room ambient illumination. Since such ambient illumination can affect the brightness of the output screen display, it is desirable to provide apparatus for maintaining a constant brightness of the output display screen regardless of changes in the room ambient light level.
To date, prior art systems have sensed the entire photocathode current of the X-ray image intensifier tube. This technique cannot control scene highlights since the total photo current is a measure of the total X-ray radiation impinging on the tube. See for example, U.S. Pat. No. 3,198,947. In another technique, in which improved operating characteristics are achieved, a photomultiplier is light-coupled by mirrors and lenses to the output window of the image intensifier tube. A dc voltage is derived from the photosensor which is used to control the X-ray generator. See for example, U.S. Pat. Nos. 2,541,187; 2,821,635; and 2,829,273. Furthermore, lenses have been used to detect light coming only from the central portion of the output window, i.e., that portion which occupies most of the radiographer's attention. See U.S. Pat. No. 3,675,020, for example.
In other, less desirable approaches, X-ray sensors which are totally separate from the image intensifier tube are used to detect and control the total amount of X-ray radiation. The sensors generally take the form of ion chambers with sensitive electrometer tubes to dc amplify the ion and electron current formed by the ionizing radiation. Thus, a point source that is very bright could produce the same control signal, and hence the same X-ray dosage, as a broad area of X-ray illumination of much reduced intensity.
The problem becomes particularly acute when the X-ray image intensifier is of the proximity focus type, that is, the X-ray input window has approximately the same diagonal dimension as the output phosphor display screen. Because such devices do not utilize minification, the brightness of the output display is lower as compared to some prior art X-ray image intensifier tubes which do use the principle of minification. Also, the output window is more directly exposed to room ambient light conditions.